1. Field of the Invention
The present invention concerns a superconductive element for conducting switching operation and, more in particular, it relates to a superconductive element using an oxide superconductor and an oxide semiconductor.
2. Description of Related Art
A field effect superconductive 3-terminal device has advantageous features, as compared with a Josephson junction device in that input-output isolation is satisfactory, switching can be conducted by a voltage signal and it can be driven by a DC power source. A field effect type superconductive 3-terminal device composed of Nb-based superconductor and Si or InAs semiconductor operating at a liquid helium temperature is described, as an example, in Physical Review Letters, Vol. 54, p. 2449, 1985. The device has such a structure that a source electrode and a drain electrode comprising a superconductor thin film are disposed on a semiconductor substrate and a gate electrode is inserted therebetween. A supercurrent penetrates from the source electrode into the semiconductor substrate by a superconducting proximity effect and flows to the drain electrode passing through the semiconductor substrate. The super current is controlled by a voltage applied to the gate electrode.
When the field effect superconducting 3-terminal device is manufactured with an oxide superconductor having a high superconducting transition temperature, the operation temperature can be made higher and a high speed operation can be expected due to the high superconducting transition temperature. However, upon actually obtaining such a device, there are the following technical problems.
When a thin film of an oxide superconductor is formed, an oxide is more desirable than Si, InAs or like other material as the semiconductor material for preventing the degradation of superconducting characteristics and reducing the contact resistance with the semiconductor, because an oxide insulation layer such as of SiO.sub.x is formed on the surface of Si or the characteristics of the oxide superconductor are deteriorated due to the reaction between Si and the compound semiconductor. A greater effect capable of suppressing such reactions can be obtained by using an oxide semiconductor layer, in particular, an oxide containing copper like that in the superconductor as the oxide superconductor layer.
However, in the oxide semiconductor, electron or hall mobility is as low as the order of 0.01 m.sup.2 /V.s. In the case of using an oxide semiconductor with such a low mobility, the coherence length is shortened, and it is further shortened in a case of operation at a liquid nitrogen temperature. In view of the above, the channel length has to be shortened further as compared with a conventional device using a metal superconductor.
However, it is extremely difficult to form a pattern of less than 0.1 .mu.m and, further, form a gate therein even with the modern pattern forming technology.
Furthermore, in a case of forming a superconductive element conducting the switching operation between the superconducting state and the normal state by using an oxide capable of exhibiting superconductivity at a high critical temperature as described above, the oxide semiconductor or oxide semiconductor employed brings about problems.
That is, there have been proposed a superconductive switching device using a crystal grain boundary of an oxide superconductor as a weak link, or a superconductive element comprising an oxide superconductor layer and a noble metal, for example, Au in contact with each other and a superconductive element using a YBa.sub.2 Cu.sub.3 O.sub.7-x film and PrBa.sub.2 Cu.sub.3 O.sub.7-x as an oxide superconductor film and an oxide semiconductor film respectively in, for example, Applied Physics letters Vol. 55, pp. 2032-2034 (for instance, ternary oxides of Y, Ba and Cu such as Y Ba.sub.2 Cu.sub.3 O.sub.7-x are collectively referred to as Y-Ba-Cu-O, while ternary oxides of Pr, Ba and Cu such as PrBa.sub.2 Cu.sub.3 O.sub.7-x are collectively referred to as Pr-Ba-Cu-O in the present specification).
This example has a structure in which a Pr-Ba-Cu-O film of about 50 nm thickness is put between two Y-Ba-Cu-O films. That is, a laminate structure of Y-Ba-Cu-O film/Pr-Ba-Cu-O/Y-Ba-Cu-O film is formed and Josephson junction characteristics are obtained with the structure described above.
However, the superconductive element as described above involves the following problems and it is difficult to apply, in particular, to superconductive 3-terminal devices.
That is, although Pr-Ba-Cu-O has an identical crystal structure with Y-Ba-Cu-O, it does not exhibit superconductivity. In R-Ba-Cu-O, if the element R in the site for Y or Pr atom is a ferromagnetic element such as Eu or Gd, R-Ba-Cu-O exhibits superconductivity with a critical temperature at 90K. However, Pr-Ba-Cu-O using Pr as R does not show superconductivity, because the magnetic interference of Pr atoms prevails over a long distance and it reaches as far as Cu-O atom plane as an electroconducting plane.
Accordingly, in a case of using Pr-Ba-Cu-O as a coupling material for a weak link device (semiconductor layer) or a normal conductive layer, the superconductive electron density is remarkably lowered at the inside of Pr-Ba-Cu-O due to the magnetic interference. Accordingly, the distance between superconductive electrodes required for the superconductive weak link device has to be decreased extremely.
In a case of a superconducting three-terminal device, if the distance between superconductive electrodes, that is, the semiconductor channel length is extremely shortened, it is difficult to form a superconductive element. For instance, in a case of a 3-terminal device which controls the supercurrent by means of the electric field effect, a gate electrode is required for applying the electric field. However, it is extremely difficult to apply the structure of a usual field effect transistor using silicon or the like in which a gate is disposed between a source electrode and a drain electrodes, to a superconducting 3-terminal device.